In a breakthrough for stem cell research, scientists at the University of Pennsylvania announced that they have successfully cultivated the stem cells that produce mouse sperm. The development promises many possible benefits - from continued fertility for male cancer patients after chemotherapy to a kind of genetic immortality.

It helps to know a bit about stem cells in general to appreciate the importance of what reproductive biologist Ralph L. Brinster and his colleagues at the University of Pennsylvania have achieved with mouse sperm stem cells.

Stem cells are non-specific parent cells that differentiate or transform themselves into various types of daughter cells, each with its own highly specific function, as Dr. Brinster explains. "And all stem cells do two things: they make another copy of themselves, so they exist forever in the body. And they also make a daughter cell which will develop into the characteristic cell of that tissue, like a nerve cell or a blood cell. But in the case of a Spermatagonial stem cell, this cell develops into a sperm. Sperm are the product of Spermatagonial stem cells," he says.

And what a lot of product it is! Every time the male heart beats, the stem cells in the testes - the sperm glands within the scrotum - produce about a thousand sperm. Each carries copies of the father's DNA, which, when combined with the DNA in the mother's egg, produces a genetically similar, yet unique offspring. It is a mechanism that makes biological diversity possible.

In 1994, Dr. Brinster and his fellow researchers successfully transplanted the sperm stem cells from a fertile mouse into the testes of an infertile mouse, which then produced the sperm of the original animal.

Today, Dr. Brinster has demonstrated that it is possible to grow sperm stem cells in a lab outside the testes and freeze them indefinitely for later use. That, theoretically at least, makes it possible to preserve the genetic line of any male forever. A race horse can father an endless variety of progeny long after it is dead, for example, and endangered species might be saved.

"If you lose a male or a male dies before puberty, you could harvest the stem cells, freeze them, and you have preserved that individual for genetic diversity even if the animal is gone! In essence, this technique, coupled with the freezing and the transplantation, makes any male germ line immortal," he says. "Biology doesn't care what you look like as long as you get that DNA transmitted to the next generation!"

According to Dr. Brinster, it will soon be possible to cultivate human sperm stem cells in a lab outside the testes. It would be an achievement with many potential benefits. "You can use the stem cells in the treatment of certain types of infertility in males. If they have a low amount of Spermatagonial stem cells in their testes, they may not make enough sperm. So you may be able to take them out, grow them up and reintroduce them. Or, for example, in men that are undergoing chemotherapy and they'll lose all their stem cells. You can take some out and freeze them - because we've show some years ago we can also freeze these - and then when the cancer is cured, you can grow them up and reintroduce them into the testes," he says.

In adult males, sometimes they freeze spermatozoa before undergoing chemotherapy. But in only about 25 percent of those cases can you recover sperm and fertilize eggs.

Dr. Ralph L. Brinster is a reproductive biologist at the University of Pennsylvania in Philadelphia Pennsylvania. He and his fellow researchers announced this week that they have successfully grown the stem cells for mouse sperm in a laboratory.